The dual layer means that it will write 9.4GB if data to special dual layer discs instead of the more usual 4.7GB.
9.4GB is a huge amount of data and it is unlikely that you will fill one of these if you are a novice.
Important things to note in a DVD writer are the writing speed - 16x is becoming the norm now - and the type of disc it will write to (+ and -) the minus type is the more popular but there is no clear winner as to which take over DVD writing as yet. Most modern writers will cope with both.
Two sites you could look at are click here and click here